Spatial–temporal evolution of ESV and its response to land use change in the Yellow River Basin, China

The value of ecosystem services, as well as their temporal and spatial characteristics, can be used to help areas develop focused and localized sustainable ecological management plans. Thus, this study conducted in the Yellow River Basin (YRB) of China, analyzed the ecosystem service value (ESV) and its spatial–temporal variation characteristics. This study used the equivalent factor and geospatial exploration methods, introduced the elasticity coefficient, and explored the response of ESV change to land-use change, based on the land use cover data from 1990 to 2020. The results showed that from 1990 to 2020, YRB ecosystem service value showed an overall increasing trend, mainly because the ecological construction project increased forest and grasslands in this region. In the past 30 years, spatial characteristics of ESV in YRB was relatively stable. The high-value areas were mainly distributed in the upper Yellow River Basin, while the low-value areas were mainly distributed in the lower Yellow River Basin, as the cold and hot spots were reduced. The ESV barycenter coordinates showed the direction of the transfer trajectory, which is first to southwest, northeast, and then to southwest. From 2000 to 2010, YRB land-use change had greater impact on ESV. Since 2010, the disturbance of ecosystem services by land-use change has decreased. Consequently, the elastic index of the upstream and Loess Plateau regions were significantly higher than that of other regions, and the impact of land-use change on ecosystem services was more obvious, due to improved large-scale ecological construction projects implementation. Conclusively, this study recommends the use of comprehensive spatial–temporal assessment of ESV for sustainable development and ecological protection in the YRB.


Materials and methods
Study area. The Yellow River originates from Yoguzonglie Basin, north of Bayan Har Mountains on the Qinghai-Tibet Plateau, and flows through Qinghai, Sichuan, Gansu, Ningxia, Inner Mongolia, Shanxi, Shaanxi, Henan, and Shandong, into the Bohai Sea in Kenli County, Shandong Province, with a total main stream length of 5464 km and 4480 m for the drop. The YRB is located between 96°-119°east longitude and 32°-42°north latitude ( Fig. 1), with a length of about 1900 km from east to west, width of about 1100 km from north to south, and a drainage area of 79.5 × 10 4 km 2 . The upper reaches of the Yellow River is located above Hekou Town, with a length of 3472 km and a drainage area of 42.8 × 10 4 km 2 ; while the middle reaches is from Hekou Town to Taohuayu, with a length of 1206 km and a drainage area of 34.4 × 10 4 km 2 ; the lower reaches is below Taohuayu, with a length of 786 km and a drainage area of only 2.3 × 10 4 km 2 . The YRB has large territory with many mountains. The height difference between the east and the west is very significant. The topography of each region varies, as well as the climate in the basin. The YRB has a large seasonal difference, with annual precipitation ranging from 200 to 650 mm in most parts, and with more than 650 mm in the south of the middle-upper, and lower reaches, particularly the northern slope of the Qinling Mountains (700-1000 mm), as it gradually increases from northwest to southeast. Precipitation is unevenly distributed, with a north-south ratio greater than 5. The YRB can be divided into 8 secondary basins and 29 tertiary basins based on its water resources zoning (slices). The basin's resident population reached 152 million people in 2020, with a regional GDP of 9642.276 billion yuan and a 46.70% urbanization rate. The entire urbanization process is accelerating, and the conflict between development and the preservation of land space is becoming increasingly serious.
Date source. The land use type data of YRB in 1990YRB in , 1995YRB in , 2000YRB in , 2005YRB in , 2010YRB in , 2015, and 2020 with a spatial resolution of 1 km was extracted from the Resource Science Data Center of the Chinese Academy of Sciences (http:// www. resdc. cn/) remote sensing monitoring database of China's land use status. The primary data source for each phase is Landsat TM/ETM remote sensing images, which are interpreted and produced through human-computer interaction. Land use is divided into six categories by the land use change classification system: cultivated land, forest land, grassland, water area, construction land, and unused land.Because the negative effects of construction land on the YRB's ecosystem far outweigh the positive effects, its ESVs were omitted. The National Agricultural Product Cost and Benefit Data Compilation in 2020 and the Statistical Yearbook of 9 provinces, including Gansu, Qinghai, and Shandong, are used to compile statistical data such as grain output and grain prices.
Evaluation methods of ESV. Ecosystem services can be classified into nine types, according to the findings of Constanza et al 5 . The "ecosystem equivalent table per unit area of ecosystems in China" proposed by Xie is the basis for this research 18 . Researchers can obtain the ESV coefficient (VC) table for each land land-use type in the YRB by correcting the YRB's socio-economic development status and the economic value of the annual natural grain yield per unit area of farmland. The specific correction process is as follows: the average yield per unit of grain is 4626.55 kg·ha −1 and the market price of grain is 0.35 USD·kg −1 in the YRB from 1990 to 2020. According to the calculation that the ecological service value per unit area of farmland is equal to 1/7 of the  14 , the equivalent factor of the ESV in the Yellow River Basin is 232.16USD·ha −1 ·a −1 , and finally they obtain the ESV coefficient of the YRB, which is showed in Table 1.
Finally, the ESV of the study area is calculated as follows: where ESV represents the value of ecosystem services (USD), S k represents the area (hectare) of the kth land use type, and VC k represents the ESV coefficient per unit area of the ecosystem type k (USD/ha).
Local spatial autocorrelation of ESV. The degree of spatial correlation of a geographic thing or phenomenon at different locations in a given region expressed using spatial autocorrelation analysis (Moran's I) 19 , can be used to determine whether the distribution of ESV has agglomeration. Hot Spot Analysis (Getis-Ord Gi*) is used to measure the aggregation and differentiation characteristics of spatial changes in ESV (ESV), and to explore whether the spatial changes of ESV have high-value agglomeration (hot spot) and low-value agglomera-  20 . It is possible to determine the location where the ESV high-value area or lowvalue area is clustered in space using hot spot analysis 21 . This was calculated using the formula: where n is the number of spatial grid units in the study area, x i and x j are the observed values of spatial unit i and spatial unit j, respectively, and ( x i − x ) is the deviation of the observed value from the mean value on the ith spatial unit, w ij is the weight matrix of space units i and j.
Land use change response resilience of ESV. The term "resilience" refers to how responsive one variable is to changes in another. This method was used to calculate the percentage change in ESV due to land use/ land cover change in this paper 22 . The formula used was: where E is the elasticity index of ESV in response to land use change; t0 is the initial stage of the study, t1 is the end stage of the study; LUP is the percentage of land use change; ∆L i represents the land use change area of i land use types, and Li represents the total area of i land use types.
The barycenter shift of ESV. ESV's centre of gravity is based on the principles of population gravity centre and economic gravity centre, and it investigates ESV's temporal and spatial evolution from space. It also examines the ESV's barycenter and change trajectory in the YRB from four perspectives: barycentric coordinates, moving direction, distance, and administrative region location in 1990,1995,2000,2005,2015, and 2020. The following formula by Liu et al. was used for the calculation 23 : Among them, M i is the attribute value of the ESV of the region in this study; X i and Y i represent the geographic coordinates, respectively, and X and Y represent the barycentric coordinates of the ESV. The barycenter area in 1990,1995,2000,2005,2015, and 2020 was calculated using the formula: where d represents the moving distance of the barycenter; (x i , y i ) and (x i+t , y i+t ) are the coordinates of the barycenter of the ESV in the ith and i + t-th years, respectively; and ρ is the conversion rate between the plane coordinates and the geographic coordinates. Generally, researchers use a constant of 111.11 km.

Results
Analysis of changes in ecosystem services value in the YRB. The results showed that from 1990 to 2020, the total ecosystem services value in the YRB showed a dynamic trend of decrease-increase-decrease, with overall increasing trend, and a total increase of 31.85 × 10 10 USD, with an average annual increase of 1.14 × 10 10 USD (Table 2). This changing trend is consistent with land use cover change in the area. In 30a, YRB cultivated www.nature.com/scientificreports/ land decreased by 8663 km 2 , due to rapid urbanization. In addition, after year 2000, China began to implement the policy of returning farmland to forest and grassland on a large scale, which accelerated the reduction of cultivated land. Results again showed that the forest area increased by 30,933,093 km 2 , indicating that the implementation of "returning farmland to forest and grassland"policy achieved great results, thus increased the value of ecosystem services generated by forest land by 167.66 × 10 10 USD. Grassland increased by 738 km 2 , as corresponding ESV increased by 28.73 × 10 10 USD, while unused land decreased by 8131 km 2 , with 9.52 × 10 10 USD ESV decrease. In general, the ecological protection and management measures in the YRB have achieved remarkable results, and ecosystem service values has been significantly improved due to forest and grassland increase.
In terms of ecosystem service structure in the YRB (Table 3), the relative proportions of various ESVs did not change significantly, resulting in relatively stable ESV structure. Soil conservation and waste disposal are the most important among them, accounting for about 37% of ESV's total value. The YRB ecosystem, as can be seen, emphasises the importance of soil conservation and waste disposal in the basin, with Climate regulation, Biodiversity conservation, and Entertainment accounting for only 11.99 percent of the total. Various services have changed to varying degrees during the study period. Waste disposal and climate regulation, for example, have suffered losses of 22.23 × 10 10 USD and 20.29 × 10 10 USD, respectively.The rest of the services showed an upward trend, among which the value of the Food production service increased the most, which was 19.03 × 10 10 USD, owing to the obvious increase of the forest land and grassland area in the YRB.

Spatial distribution and variation characteristics of ecosystem services in the YRB. The total
ESV value of the study area and changes in the value of each service could not reflect their spatial differences. To describe the temporal and spatial distribution pattern of ESV in the study area, the natural breakpoint method was used. This method was further used to classify ESV with reference to existing studies, and divided the area into four levels: low-value, lower-value, higher-value, and high-value areas. Takin the three-level watershed of the YRB as the statistical unit for analysis, the result showed that the higher the level, the higher the ESV. As shown in Fig. 2, from 1990 to 2020, the spatial characteristics of ESV were relatively stable. The YRB's upper reaches, from Shizuishan to the north bank of Hekou Town, the Fenhe River Basin, from Hekou Town to Longmen, and the Jinghe River Basin are all rich in high-values. The forest and grassland are relatively concentrated in the above-mentioned areas, the ESV coefficient is high, and the watershed area is large, resulting in a high total ESV. The higher value areas are mainly distributed in the areas from Longyang Gorge to Lanzhou main stream basin, the Daxia River and Tao River basin, and the Wei River basin. The area above Baoji Gorge and the inflow area fall in the transition zone between the high-value area and the lower-value area. For example, the transition area between the Loess and Qinghai-Tibet Plateaus is a higher-value area. The lower-value area mainly includes the Huangshui River Basin, the Datong River Basin, the basin below Lanzhou, and the Guanzhong Plain area. Thus, the unused land in this area is widely distributed. Due to the large area of construction land in the Guanzhong Plain, the ecosystem service value has shrunk. The low-value area is found in the YRB's lower reaches, which contains the most extensive and large area of construction land in the basin, has a poor ecosystem service function, and is also the YRB's most economically developed area. In terms of changes in the value of watershed ecosystem services, the number of watersheds at the ESV level did not change significantly between 1990 and 2020. The average ESV of the watershed is 40.52 × 10 10 USD. There were 7 high-value, 5 higher-value, 12 lowervalue, and 5 low-value watersheds respectively.
The hotspot analysis revealed the spatial agglomeration characteristics and ESV evolution law in the YRB from 1990 to 2020 (Fig. 3). In most of the YRB, the ESV accumulation characteristics were not significant in space, and significant areas were dominated with high and low ESV accumulation. The Maqu-Longyangxia River Basin, Daxia River and Tao River Basin, the Datong River Basin, and Fen River Basin were the five core areas where ESV had the highest value. The Inner River, YRB's northern and eastern margins, and the lower reaches are primarily low-value agglomeration areas. The high-value agglomeration area and low-value agglomeration area did not change significantly in space from 1990 to 2020, but the number of grids in each decreased from 647 to 627 and 699 to 681, respectively. In general, the YRB's high-value agglomeration areas are strewn about, whereas the low-value agglomeration areas are scattered.  (Fig. 4). During the study period, the ESV barycenter coordinates showed a transfer trajectory of first to southwest, then to northeast, and then to southwest. From the perspective of overall transfer direction, ESV barycenter shifted from northeast of Huanxian County to southwest from 1990 to 2020. The ESV in the northeast decreased, while that in southeast increased. From 1995 to 2000 and from 2000 to 2005, the migration distance of ESV barycenter in the YRB was longer by 16.33 km and 15.75 km, respectively, while the barycenter migration distance of ESV from 2005 to 2020 was shorter.  The range of changes in land land-use types was wide during this time, but the average elasticity index was low because there were so many different types of land land-use changes, such as the conversion of forest land and cultivated land to construction land, and the conversion of forest land and water area to cultivated land. The decrease in ESV caused by the change in land use per unit area was minor. Furthermore, the forest land and grassland in the river basin have been effectively increased, as ESV has increased. Overall, the value of ecosystem services has remained relatively constant. Accurate spatial statistics on the elasticity index from 1990 to 2020 was carried out (Fig. 5). The elastic index of the upper YRB and Loess Plateau is higher, and the impact of land use change on ecosystem services is more apparent in this region, according to the findings. This is mainly due to the implementation of large-scale ecological engineering measures in response to vegetation degradation in the upper reaches of the YRB and soil erosion in the middle reaches (Loess Plateau), by the Chinese government. In addition, Lanzhou New District, Guanzhong Plain, and the lower Yellow River region also showed higher elasticity index. The above-mentioned regional development and construction, as well as human activities, have resulted in a rapid increase in construction land, resulting in a significant decline in ecosystem services and a higher resilience index as a result of rapid urbanisation.   www.nature.com/scientificreports/ In general, the land use types in the YRB have changed dramatically, and land type conversion is very common. The conversion of ecological land to urban construction land, as well as the conversion of unused and cultivated land to ecological land, has resulted in significant changes in ecosystem service value. This demonstrates that the basin's ecological construction projects have yielded positive environmental results.

Discussion
Different scholars use different evaluation index systems and calculation methods to calculate the value of ecosystem services, according to literature reports. Yin et al. found that the ESV in the YRB showed a dynamic change from 1990 to 2020, first increasing and then decreasing, with an overall increasing trend, which is consistent with the trend of this study 24 . However, the accounting methods differ due to the different evaluation index systems chosen, resulting in numerical differences. The ESV of the YRB is calculated using the unit area value equivalent factor method in this study. This method has a relatively simple and comprehensive evaluation system, and it has proven to be a more effective method than evaluating benefits for evaluating ecosystem services value in larger areas. The grain yield and price in each county were not used in the calculation of the equivalent factor due to lack of data 25 . The accuracy and adaptability of ESV must be ensured during the evaluation process. More time-sensitive and local ecological indicators, as well as high-precision remote sensing data, should be incorporated into the evaluation system, as the relationship between humans and the natural environment continues to evolve; considering the locality of ESV research, more time-sensitive and local ecological indicators and high-precision remote sensing data should be incorporated into the evaluation system; the ESV evaluation model can fully consider ecological processes and will become the future research direction. Some scholars have found that the reduction of farmland and forest area can lead to ESV decline 26 . Due to the fact that urban construction land tends to occupy high-quality farmland, forest land, and grassland, the loss of ESV will result from the reduction of these land types. ESV decreased from 1990 to 1995 as forest land and water area decreased. ESV increased steadily from 1995 to 2010, owing primarily to increases in forest land and water area. ESV decreased from 2010 to 2020 as a result of policies such as "returning farmland to forests," "natural forest protection plan," and "Three North Protection Plan" 27 . Following 2010, as social and economic levels improved and urban development accelerated, construction land for residences, industrial parks, and public service facilities increased significantly, while grassland and cultivated land decreased, resulting in a decrease in ESV. The findings of the study agree with those of 28 .
The hot and cold spot analysis method was used to characterise the spatial difference of ESV in this study. Many researchers have used it to investigate the spatial distribution of ESV 29 . Because the economy of upstream areas like Qinghai, Lanzhou, and Sichuan lags behind that of downstream areas, there is less construction land, grasslands are more widely distributed, and these areas have higher altitudes and lower accessibility to human activities. The year-round water supply provided by melting snow and ice nourishes the local grassland and primarily performs ecological service functions such as soil and water conservation 30,31 . The grassland forest resources of the northern Qilian Mountains, Taihang Mountains, and Qinling Mountains are abundant and hotspots like mediation, habitat, and human services are concentrated, and designated as ecological protection barriers. This ensure regional ecological security, define YRB's ecological corridor protection areas, create a system of nature reserves, and fully utilise the regulating role of ecological barriers. Cold spots, such as the Mu Us Sandy Land and Loess Plateau, are mostly found in YRB's middle and lower reaches. These areas are crisscrossed by ravines, hills, and beams, and have a fragile ecological environment, low forest and grass coverage, severe soil erosion, mostly unused and cultivated land, and weak ecosystem services, resulting in low ESV. There was flat terrain, convenient transportation, developed economy, and a lot of construction lands in downstream provinces like Henan and Shandong, resulting in ESV loss, which is represented by reduced biodiversity, fragmented habitat distribution due to land use changes, and species on the verge of extinction 32 . The fragmentation of habitat distribution caused by land use change puts species at risk of extinction. In this regard, the low-yield and poorquality arable lands in the region are included in the new round of the project of returning farmland to forest and grassland (sand control and afforestation, planting grass, and eliminating desertification). Simultaneously, the gravity model was used to investigate the YRB's changing trend of ESV's "centre of gravity." The temporal and spatial changes in ESV, as well as changes in ESV over time, were analyzed for ecological impact and government decision-making. This is helpful in identifying problems and providing support for the formulation of ecological and environmental protection policies.
The results of this study revealed that water area contributed the most to total ESV. However, in recent years, due to accelerated urbanization, sharp increase in water consumption, discharge of industrial sewage, and the use of chemical fertilizers and pesticides, water pollution has been severely aggravated. Despite government's creation of "Transboundary Water Body Joint Protection Action" to address the shortcomings of the environmental protection law, penalties for illegal sewage discharge remain insufficient, and water pollution remains a serious problem. Therefore, the state's penalty for violations should be increased. Between 1990 and 2020, total ESV in YRB increased, owing largely to policies implemented by the national government, such as the conversion of farmland to forest and farmland to grassland. These policies made significant contributions to the promotion of ESV in YRB and security of regional ecological assets, as well as improving regional ecosystem, thus ensuring long-term ecosystem services benefits. Simultaneously, frequent human activity and expansion of urban construction land, renders large portions of cultivated lands occupied. As a result, cultivated land not only serves as an ecologically returning farmland and a protector of the environment, but also makes significant sacrifices in the name of social and economic development.As a result, maximising the safety of cultivated land remains critical in the process of regional land development and utilisation. Several suggestions were made in response to this: first, strengthen protection of basic farmland, check the borders of cultivated land, and balance cultivated and uncultivated lands; second, ESV should be included in decision-making when formulating regional land www.nature.com/scientificreports/ use planning policies; third, efficient management of returning farmland to forest and grassland project. Fourth, planning construction lands, avoiding disorderly development of construction lands, evaluating economic and intensive use of construction lands, digest and revitalise stock land, and improve the efficiency of urban land use in land use planning. Furthermore, because the upper, middle, and lower reaches of YRB are the ecological, energy, and economic centres 33 , the proposal of ecological protection countermeasures in YRB must classify the river basin according to the different protection priorities of the region and adapt measures to local conditions in order to strive for ecologically balanced development of human-environment relationships.

Conclusion
This study used the equivalent factor method to evaluate the ESV of the YRB, revealing the numerical changes and spatial distribution characteristics of the ESV in the YRB, and delving into the ESV's response to changes in land use types. The following are the conclusions drawn: (1) In terms of time, from 1990 to 2020, the ESV in YRB showed an overall increasing trend, with total increase of 31.85 × 10 10 USD, and average annual increase of 1.14 × 10 10 USD. The ecosystem service value increased by 167.66 × 10 10 USD and 28.73 × 10 10 USD; In space, from 1990 to 2020, the spatial characteristics of ESV were relatively stable. The high-value areas were mainly distributed in the upper reaches of the YRB, from Shizuishan to the north bank of Hekou Town, the Fenhe River Basin, Hekou Town to Longmen, and the Jinghe River Basin; the low-value areas were distributed in the lower reaches of the YRB. The ESV of the YRB has the characteristics of spatial agglomeration in space. The barycenter coordinates of the ESV show a transfer trajectory of first southwest, then northeast, and then southwest.

Data availability
All data generated or analyzed during this study are included in this published article.